Insulation coordination 1

[chuckd83]

I have a surge arrester at the 69 kV transformer bushing which adequately protects it (per IEEE C62.22-2009 Section 5.2). The transformer is connected to GIS 30 feet away by overhead line. I'm trying to determine if the GIS is protected by the same arrester. Can this be done without EMTP? I'm looking through the same standard Annex C and I'm lost. Can I even use it as the first line says "This annex provides a simplified procedure for calculating acceptable separation distances for simple air insulated substations..."

 

[Power0020]

The GIS will have a different surge impedance (~ 90 ohm) compared with AIS (350~450 ohm), there will be reflections at the SF6/air bushing, you can calculate the reflected voltage using impedance discontinuity formula.

you should first calculate the voltage at 30 feet away from SA, this will be higher than the SA voltage depending on surge steepness.

This will help getting a figure of the voltage at SF6/air bushing, you would need to perform another calculation at the other extremities (cables, transformer, bushings...etc)connected to the GIS.

I personally prefers to use EMTP to get better accuracy

 

[chuckd83]

"you should first calculate the voltage at 30 feet away from SA"

The SA is AT the transformer bushing. The air bushing for the entrance into the GIS building is 30 feet away:
Transformer 69kV bushing - SA - 30 ft. overhead line - air bushing - GIS.

The diagrams in Annex C don't make any sense given my situation:
Lightning stroke - kilometers of transmission line - switch - SA - transformer

Nor do the equations to calculate S' - rate of rise of incoming surge on the transmission line:

S' = Kc / dm where dm is supposedly miles. I have no idea what value to even use for dm in my situation, but it has to be incredible short. Making S' skyrocket and therefore Dt (max distance between GIS and SA) incredible small.

 

[chuckd83]

bump

 

[Power0020]

see the attached file for calculations,

the steepness cause the voltage at transformer to be higher than SA depending on the distance between them (separation effect)

the formula is : Ut= Usa+2Sl/v

v is the propagation speed, about 3 x 10^8 m/s
S is the steepness, about 1000 kv/us

 

[chuckd83]

Thanks for the reply. "S is the steepness, about 1000 kv/us". S is what I'm trying to calculate. Annex C.5, S' = K / d where d is multiples of span length (i.e. miles). My span is maybe 30 ft, if that. S' will be huge, not "about 1000 kv/us."

Does the annex allow me to just use 1000 kv/us?

 

[chuckd83]

The previous standard (1997) was just 11 x MCOV, simple.

 

[Power0020]

steepness does affect the results, it differs from a country to another, first time to see 11 x MCOV but seems easy.

steepness is just that slope of impinging wave, not too simple to calculate as it depends on the striking current with some probability distribution.

I remember I have seen a formula in ABB document, will let you know if i got it.

 

[Marmite]

This is a useful document. It may be the one Power0020 is referring to.

Regards
Marmite

 

[chuckd83]

C62.22-1997 Annex C:
"S' is the rate of rise of incoming surge on the transmission line (kV/us) (Use 11 kV/us per kV MCOV rating to a maximum of 2000 kV/us - IEEE Std C62.11-1993)"

It seems without software 11 x MCOV is my only option.

 

[prc]

Power0020,From which book(Title &Author) you have the exhibit?

 

[Power0020]

Handbook of Power System Engineering


Yoshihide Hase

ISBN: 978-0-470-03366-1

574 pages

June 2007